Conventional optical communication systems are known in which light having a single wavelength is modulated in accordance with an information stream and transmitted along an optical communication path, such as an optical fiber. In order to increase the information carrying capacity of such systems, so-called wavelength division multiplexed (WDM) optical communication systems have been developed. In a WDM optical communication system, multiple modulated optical signals, each having a different wavelength, are transmitted on the optical communication path. WDM optical communication systems often include optical transmitters, which output the optical signals, and an optical combiner, which combines the optical signals into a WDM optical signal that is supplied to one end of an optical communication path. At the receive end of the optical communication path, the optical signals may be de-multiplexed and supplied to corresponding optical receivers.
In order to further increase the capacity of WDM optical communications systems, optical signals having different polarizations, but the same wavelength, are modulated independently of each other and combined or polarization multiplexed onto the optical communication path. Thus, optical signals, at each wavelength, can have first light with a first polarization and second light with a second polarization, and the first and second light may be modulated to carry separate information streams. As generally understood, the first polarization component may have a transverse electrical (TE) polarization and the second polarization component may have a transverse magnetic (TM) polarization, such that the TE polarization is oriented in an orthogonal direction relative to the TM polarization.
WDM optical communication systems may be assembled from discrete components, wherein, for example, the transmitters and combiners are housed separately from one another or provided or mounted on a board or card. Alternatively, photonic integrated circuits (PICs) have been developed in which these components, as well as others, are integrated on a common semiconductor substrate. In order to realize further capacity increases, modulated optical signals generated by a PIC may be polarization multiplexed, as noted above.
In particular, each of the optical transmitters on the PIC may include a laser, and portions of the light output from the laser may be separately modulated to provide first and second modulated optical signals having the same wavelength. Since, as generally understood, the laser typically outputs light having a TE polarization, the polarization of one of the first and second modulated optical signals may be rotated by a polarization rotator to have TM polarization while the other modulated optical signal is not rotated and remains at a TE polarization. The first and second modulated optical signals (also referred to herein as TE and TM modulated optical signals, respectively) may then be combined in a polarization beam combiner provided either on the PIC or off the PIC.
In one example, the PIC includes one or more waveguides which direct the TE polarized light portions supplied by the laser from one component on the PIC to another prior to rotation and polarization multiplexing. For example, a waveguide may be provided that routes or directs lght from the laser to the combiner. A modulator may be provided between the laser and combiner, which may also include waveguides. In addition, known combiners, such as arrayed waveguide gratings may further include waveguides.
The waveguides on a PIC may include both straight and bent or curved portions in order to conform to a device layout, for example. The straight and curved portions of the waveguide have different radii of curvature (“ROC”), such that there is an abrupt change in the ROC (i.e., a discontinuity) where a straight section joins a curved section, for example. Many such ROC discontinuities may occur over the length of a waveguide. If this “ROC profile” contains large enough abrupt changes or discontinuities in ROC, it can induce polarization scattering from the desired TE light to undesirable TM light. The scattered light from the multiple scattering events can add up as light propagates down the length of the waveguide. Because the scattered light remains coherent (trapped in the TM mode of the waveguide), how the various components add up will depend on their phase relationships.
U.S. Patent Application Publication 2012/0002920, the contents of which are incorporated herein by reference, describes adjusting the phase relationships of the various scattered TM light components to destructively interfere with each other, thereby minimizing the combined scattering to TM light. In short, the previously described method involves adjusting the physical lengths of the various arcs in the waveguide to be an integer multiple of the so-called “TE-TM beat length” of the waveguide.
The TE-TM beat length of the waveguide is simply λ/ΔN, where λ is the free-space wavelength of light propagating in the waveguide, and ΔN is the difference between the effective refractive index (neff) of the fundamental TE and TM modes of the waveguide (i.e. ΔN=neffTE−neffTM). ΔN is known in the art as the “birefringence” of the waveguide. High birefringence means the TE and TM fundamental modes travel at very different speeds. Low birefringence means the TE and TM modes travel at very similar speeds. The beat length physically represents the length of waveguide required to reproduce the phase relationship between TE and TM light. In other words, if TE and TM light in the fundamental modes of the guide have a given phase relationship at one location, as they propagate down the waveguide, the phase relationship will drift due to the light propagating at different speeds in the guide. At one beat length down the guide, the phase drift will equal 2π radians, and hence, the phase relationship will be the same as it was one beat length earlier.
A curved section that is an integer multiple of beat lengths is like having no curved section at all, thereby eliminating scattering to TM light. Physically, if the curved section arc length is an integer multiple of beat lengths, the TM light scattering at the entrance and exit of the curved section will add destructively, cancelling out or minimizing the combined TM light scattering. Likewise, if the curved section is an odd integer multiple of half a beat length, the entrance and exit scattered TM light components will add constructively leading to “resonant TM polarization scattering.”
In the US Patent Application Publication noted above, arc lengths are set to integer multiples of the beat length to minimize TM light scattering, and avoid the undesirable resonant TM polarization scattering condition. However, it is often the case that the geometry of the waveguide circuit is constrained to specific dimensions. In such cases, the arc may not be set to the desired length. One example of such a constrained arc length scenario is multiple parallel waveguides that bend together, where the inner and outer arc lengths are necessarily different from each other. In this case, it is not possible to individually optimize the arc length of each waveguide. Therefore, there is a need to optimize curved sections to avoid the resonant TM polarization scattering condition without adjusting the arc lengths.